The present invention relates to protecting devices in input/output cells from excessive input voltages.
A goal in integrated circuit manufacturing is to increase circuit density and functionality. Accordingly, there has been a great deal of effort put into reducing the size of individual transistors so that more transistors, and thus more functionality, can be placed on each device.
But there is a downside to these higher densities and smaller devices. For example, smaller devices can only withstand a limited voltage before oxide breakdown occurs. Also, higher densities can result in an increase in power supply dissipation per unit area of an integrated circuit, which can limit operability and lifetime (as measure in mean time before failure) of the circuit. To mitigate both these consequences, the power supply voltages of integrated circuits has been progressively lowered over the years, from 5 volts to 3.3, then to 2.5 and recently 1.8 volts and even lower.
Not all integrated circuits operate at these lower voltages, however. That is, many devices have been designed to operate at 3.3 volts, 5.0 volts, or at even higher voltages. This means that some integrated circuits operating at a lower 1.5 or 1.8 supply voltage need to accept input signals having higher voltages. For example, a device operating with core or internal voltage supplies of 1.8 volts and ground may need to receive signals that switch between 5.0 volts and ground.
This arrangement leads to over-voltage problems for output drivers, input receivers, and input/output (I/O) cells. For example, when receiving an input signal, if the received voltage exceeds the gate-oxide breakdown voltage of the input devices, the devices may become stressed and damaged.
Thus what is needed are circuits, methods, and apparatus for protecting the devices in input and output structures from input voltages that exceed the device's oxide breakdown voltage.